Prior art automatic tachyarrhythmia detection systems for automatic cardioverter/defibrillators rely upon the presence or absence of electrical and mechanical heart activity, such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement, for example, and/or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
In pacemaker/cardioverter/defibrillators presently in clinical evaluation, fibrillation is distinguished from ventricular tachycardia using rate based criteria, In such devices, it is generally common to specify the rate or interval ranges that characterize a tachyarrhythmia as opposed to fibrillation. However, some patients may suffer from ventricular tachycardia and ventricular fibrillation that have similar or overlapping rates, making it difficult to distinguish low rate fibrillation from high rate tachycardia. In addition, ventricular fibrillation may display R-R intervals which may vary considerably, resulting in intervals that may fall within both the tachycardia and fibrillation rate or interval ranges, or outside both.
Current pacemaker/cardioverter/defibrillator arrhythmia control devices, such asthose commercially available from Medtronic, Inc., typically employ programmable fibrillation interval ranges and tachycardia detection interval ranges which are adjacent to one another. In the Medtronic devices in particular, the interval range designated as indicative of fibrillation consists of intervals less than a programmable interval (FDI) and the interval range designated as indicative of ventricular tachycardia consists of intervals less than a programmable interval (TDI) and greater than or equal to FDI. R-R intervals falling within these ranges are measured and counted to provide a count (VTEC) of R-R intervals falling within the tachycardia interval range and a count (VFEC) of the number of intervals which fall within the fibrillation interval range, out of a preceding series of a predetermined number (FEB) of intervals. VTEC is incremented in response to R-R intervals that are greater than or equal to FDI but shorter than TDI, and is reset to zero in response to intervals greater than or equal to TDI and is insensitive to intervals less than FDI. VTEC is compared to a programmed value (VTNID) and VFEC is compared to a corresponding programmable value (VFNID).
Further, when one of the counts equals its corresponding programmable value, the device diagnoses the presence of the corresponding arrhythmia, i.e. fibrillation or tachycardia and delivers an appropriate therapy, e.g. anti-tachycardia pacing, a cardioversion pulse or a defibrillation pulse. In addition, the physician may optionally require that the measured R-R intervals meet a rapid onset criterion before VTEC can be incremented and can also optionally require that should a rate stability criterion fail to be met, VTEC be reset to zero. This detection system has proven effective in distinguishing between fibrillation and ventricular tachycardia so that appropriate therapies may be delivered. However, an increased level of accuracy in classifying rhythms having intervals close to FDI is believed desirable. In addition, the ability to provide a separate therapy set for fast tachycardias as opposed to slower tachycardias is also desirable.